This invention relates to foams made by photopolymerizing emulsions. The emulsions comprise a reactive phase and an immiscible phase wherein the reactive phase or both phases are continuous. The resulting foams may be closed or open cell, depending on the initial emulsion microstructure.
In the past, thermal polymerization has been used as a technique to polymerize high internal phase emulsions (HIPE)s. Typically these HIPEs contain styrene and divinylbenzene as well as other monomers. The thermal polymerization technique is very time intensive, usually requiring more than 10 hours for polymerization, and prohibits continuous production of foams. For thermal polymerization, emulsions are made and poured into a sealed container in which they are heated for many hours. After polymerization, the solid foams (still containing water) are removed and dried in an oven.
The emulsions used in thermal polymerization must remain stable for many hours until the polymerization process is complete, otherwise inhomogeneous foam materials would be formed. The requirement for a stable emulsion limits the types of monomers and surfactants that may be used in a thermal polymerization process.
The present invention features a novel method for creating foams, including open cell foams, from water-in-oil emulsions. The applicants found, surprisingly, that they could make foams from high internal phase emulsions (HIPEs) and other water-in-oil emulsions using a photopolymerization process. This is unexpected because emulsions typically have an opaque appearance and would not be expected to transmit enough light to conduct a photopolymerization reaction. Applicants found that they could cure an emulsion as thick as 8 millimeters.
The foams may be made by a batch process, or a continuous process in which the emulsion may be coated on a moving support. In either case, the foam is polymerized and crosslinked by exposure to actinic radiation. Some embodiments of the foams may be polymerized and crosslinked within one second or less of radiation illumination time. The fast polymerization process of the present invention allows a broad range of materials to be used because the emulsion needs to be stable for only a short time (seconds to minutes).
One aspect of the present invention provides a process for making a crosslinked polymeric foam comprising: a) mixing a reactive phase comprising at least one polymerizable material, at least one crosslinking agent, and at least one emulsifier with at least one photoinitiator and a liquid fluid immiscible with the reactive phase to form an emulsion wherein the immiscible fluid forms a discontinuous or co-continuous phase with the continuous reactive phase; b) shaping the emulsion; and c) exposing the emulsion to actinic radiation to form a crosslinked polymeric foam containing residual immiscible fluid.
The process may comprise further steps of exposing the emulsion to heat and/or removing residual immiscible fluid from the foam.
The polymerizable material may be ethylenically- or acetylenically-unsaturated, such as an acrylate, and free-radically or cationically-curable. The polymerizable material may be the same as the crosslinking agent or the emulsifier.
The immiscible phase is typically water, but may comprise other liquids such as fluorocarbons or organic liquids. The immiscible fluid may comprise 74 volume percent, or more, of the emulsion.
The reactive phase may include, e.g., non-polymerizable materials and materials that can incorporate functional groups into the foam.
The structure of the foam of the present invention may be controlled by aging the emulsion prior to polymerization or by selection of a particular agitation method for making the emulsion.
The emulsion may include photoinitiators in the reactive or immiscible phase. Preferably, the photoinitiators are activated by ultraviolet or visible radiation of 300 to 800 nanometers.
Polymerization and crosslinking of the emulsion may occur in as little as 10 minutes or even 10 seconds.
A further aspect of the invention is an emulsion having a continuous reactive phase comprising at least one polymerizable material and at least one crosslinking agent, a discontinuous or co-continuous phase comprising a liquid fluid immiscible with the reactive phase, and at least one photoinitiator.
A further aspect of the invention is an open cell cross-linked foam comprising no thermal initiator residue. Another aspect of the invention is an open cell cross-linked foam comprising residue of a photoinitiator that absorbs at a wavelength of 300 to 800 nanometers.
A further aspect of the invention is a closed cell cross-linked foam comprising no thermal initiator residue. Another aspect of the invention is a closed cell cross-linked foam comprising residue of a photoinitiator that absorbs at a wavelength of 300 to 800 nanometers.
The foams may be crosslinked within the voids of a material selected from the group consisting of polymeric, woven, nonwoven, and metals. Alternatively, the foam may contain non-polymerizable materials selected from the group consisting of polymers, metals, particles, and fibers.
Some foams of the present invention can absorb at least two and one-half times their weight in fluid. Some of the foams collapse when fluid is removed.
Another aspect of the present invention is articles made using the foams of the present invention.
Foams of the present invention made from HIPEs have relatively homogeneous structures and may possess cell sizes between 1 and 200 microns and densities of at least 0.01 g/cc. Cells are typically joined by open xe2x80x9cwindowsxe2x80x9d or holes connecting adjacent cells. Some of the resulting foam materials may be capable of absorbing 25 or more times, typically 4 to 16 times, their weight in fluid (water or organic fluids). Some of the foams are extremely porous, having Gurley values (at 50 cc of air) of 2 to 70 seconds for a 0.2 cm (80 mil) thick specimen.
Foams of the present invention made from non-HIPE emulsions typically have interconnecting channel structures rather than a well-defined cellular structure.
Closed cell foams may also be made using the photopolymerization process of the present invention.
As used in this invention:
xe2x80x9cHIPExe2x80x9d or xe2x80x9chigh internal phase emulsionxe2x80x9d means an emulsion comprising a continuous reactive phase, typically an oil phase, and a discontinuous or co-continuous phase immiscible with the oil phase, typically a water phase, wherein the immiscible phase comprises at least 74 volume percent of the the emulsion;
xe2x80x9cwater-in-oil emulsionxe2x80x9d means an emulsion containing a continuous oil phase and a discontinuous water phase; the oil and water phases may be co-continuous in some cases;
xe2x80x9creactive phasexe2x80x9d means the continuous phase which contains the monomer or reactive species that are sensitive to reactive propagating species (e.g., those having free radical or cationic centers) and can be polymerized or crosslinked;
xe2x80x9cimmiscible phasexe2x80x9d means a phase in which the reactive components have limited solubility; the immiscible phase may be discontinuous, or co-continuous with the reactive phase;
xe2x80x9cstablexe2x80x9d means the composition and microstructure of the emulsion is not changing over time;
xe2x80x9cfunctional groupxe2x80x9d means a chemical entity capable of undergoing a non-polymerization reaction;
xe2x80x9cmonomerxe2x80x9d means chemical species capable of polymerizing, it includes monomers and oligomers;
xe2x80x9creactive surfactantxe2x80x9d means a surfactant (i.e., emulsifier) having sufficient reactivity to undergo polymerization reactions such that it becomes part of a polymer backbone;
xe2x80x9copen cellxe2x80x9d means a foam wherein the majority of adjoining cells are in open communication with each other; an open cell foam includes foams made from co-continuous emulsions in which the cell structure is not clearly defined, but there are interconnected channels creating at least one open pathway through the foam;
xe2x80x9cwindowxe2x80x9d means an intercellular opening;
xe2x80x9cshapingxe2x80x9d means forming into a shape and includes pouring, coating, and dispensing;
xe2x80x9cpolymerizexe2x80x9d or xe2x80x9ccurexe2x80x9d are used interchangeably in this application and indicate a chemical reaction in which monomers, oligomers, or polymers combine, including by crosslinking, to form a chain or network;
xe2x80x9ccrosslinkingxe2x80x9d means the formation of chemical links between polymer chains;
xe2x80x9ccrosslinking agentxe2x80x9d means a material that adds to a polymer chain a site capable of forming a link to another polymer chain;
xe2x80x9ccationically curable monomerxe2x80x9d means a monomer capable of undergoing polymerization in which cationic species propagate the polymerization reaction and includes monomers containing, e.g., epoxide or vinyl ether moieties;
xe2x80x9cethylenically unsaturatedxe2x80x9d means a monomer having a carbon-carbon double bond in its molecular structure;
xe2x80x9cacetylenically unsaturatedxe2x80x9d means a monomer having a carbon-carbon triple bond in its molecular structure;
xe2x80x9cactinic radiationxe2x80x9d means photochemically active radiation including near infrared radiation, visible light, and ultraviolet light;
xe2x80x9cUVxe2x80x9d or xe2x80x9cultravioletxe2x80x9d means actinic radiation having a spectral output between about 200 and about 400 nanometers;
xe2x80x9cvisible lightxe2x80x9d means actinic radiation having a spectral output between about 400 to about 800 nanometers;
xe2x80x9cnear infraredxe2x80x9d means actinic radiation having a spectral output between about 800 to about 1200 nanometers;
xe2x80x9cphotoinitiatorxe2x80x9d means a chemical added to selectively absorb actinic radiation and generate reactive centers such as free radicals and cationic species;
xe2x80x9cthermal initiatorxe2x80x9d means a species only capable of efficiently inducing or causing polymerization or crosslinking by being exposed to heat;
xe2x80x9chomogeneous compositionxe2x80x9d means having a uniform distribution of chemical components when examined on a scale of 0.5 micrometers;
xe2x80x9cpressure sensitive adhesivexe2x80x9d or xe2x80x9cPSAxe2x80x9d means an adhesive that will adhere to a variety of dissimilar surfaces upon mere contact without the need of more than finger or hand pressure; PSAs are sufficiently cohesive and elastic in nature so that, despite their aggressive tackiness, they can be handled with the fingers and removed from smooth surfaces with little or no residue left behind; PSAs can be quantitatively described using the xe2x80x9cDahlquist criteriaxe2x80x9d which maintains that the elastic modulus of these materials is less than 106 dynes/cm2 at room temperature. See Pocius, A. V., Adhesion and Adhesives: An Introduction, Hanser Publishers, New York, NY, First Edition, 1997, and
xe2x80x9cvoidxe2x80x9d means any open space, in a foam, such as holes, cells, and interstices.
An advantage of at least one embodiment of the present invention is that the photopolymerization process may be completed in seconds as opposed to thermal polymerization methods that typically require many hours.
An advantage of at least one embodiment of the present invention is that the faster polymerization process allows the use of emulsion compositions that cannot remain stable for the length of time required to complete thermal polymerization.
An advantage of at least one embodiment of the present invention is that a broad spectrum of foam physical properties can be generated by manipulating the type of monomers and co-monomers, the monomer to co-monomer ratio, cell size, percentage of open cells, density of the foam, and mixing methods.
An advantage of at least one embodiment of the present invention is that the process allows continuous foam production as opposed to the batch processing generally required with thermal polymerization of water-in-oil emulsions.
An advantage of at least one embodiment of the present invention is that salts in the water phase of the emulsion are not necessary to provide lengthy stability during emulsification and polymerization. This also eliminates the need to wash away excess salts after polymerization.
An advantage of at least one embodiment of the present invention is that thin foam articles can be produced directly by the present method as opposed to having to cut thin articles from the products of a batch thermal polymerization process.
An advantage of at least one embodiment of the present invention is that the foams may be hydrophilic when produced, depending on monomer and surfactant choice. This eliminates having to incorporate hydrophilizing agents or treat the foam surfaces to make them hydrophilic (e.g., when used as an absorbent) as is required with some thermally polymerized foams.
An advantage of at least one embodiment of the present invention is that foams having a wide range of cell and window sizes can be obtained because the method of the present invention allows foams to be made from emulsions that are stable for as little as one minute or less.
An advantage of at least one embodiment of the present invention is that the foam materials are suitable for a myriad of applications such as energy and fluid absorption, insulation, and filtration. An advantage of at least one embodiment of the present invention is that multilayer articles comprising one or more foam layers may be made.
An advantage of at least one embodiment of the present invention is that articles comprising regions, i.e., areas, having foams that differ in composition or density may be made.
An advantage of at least one embodiment of the present invention is that the foams may be made by a continuous process.